Russel R. Hahn,
Department of Crop and Soil Sciences, Cornell University
Two thistles common to New York State are bull thistle (Cirsium vulgare) and Canada thistle (Cirsium arvense). Both were introduced from Eurasia and became naturalized in Canada and the United States. Although closely related and somewhat similar in appearance, theses thistles exhibit some differences in form and have very different life cycles. These life cycle differences play an important role in timing of control measures.
Bull thistle is a biennial weed that reproduces by seed only. All biennials require two growing seasons to complete their life cycle. In the first year, bull thistle germinates from seed and forms a rosette or basal cluster of leaves (see photo) with a large fleshy taproot. After overwintering in this stage, the plants complete their life cycle by forming a flowering stalk and setting seed during the second growing season. The stems of bull thistle may be 3 to 6 feet tall, are often branched, and are more or less hairy. The leaves are deeply cut, spiny, and run down the stem (Figure 1). Deep purple or rose flower heads are formed during the second growing season. These heads are 1 to 2 inches in diameter and are surrounded by numerous spiny tipped bracts. Bull thistle is found in pastures, meadows, and waste areas. Although it is an aggressive weed in these situations, it does not survive in tilled fields.
Canada thistle is a perennial weed that reproduces by seeds and horizontal roots. These roots extend several
feet deep, some distance horizontally (Figure 2), and allow individual plants to live for more than two years. Canada thistle stems are grooved and are 2 to 5 feet tall with branching only at the top. The stems are somewhat hairy when mature. The leaves are smooth, somewhat lobed, and usually have crinkled edges and spiny margins (see photo). The flower heads are numerous, compact, and are borne in clusters. The lavender heads are ¾ inch or less in diameter. Male and female flowers are usually in separate heads and on different plants. As a result, some patches of this weed never produce seed. Canada thistle is found throughout the northern half of the United States. Like bull thistle, it can be problematic in pastures, meadows, and waste areas. In addition, its’ perennial nature allows it to thrive in cropland as well.
Both of these thistles are somewhat sensitive to growth regulator herbicides (synthetic auxin/Group 4 herbicides) such as 2,4-D and Banvel/Clarity. These readily translocated herbicides are recommended for control or suppression of both species in grass pastures, however application rates and timing differ.
For bull thistle control, application of 3 pt/A of 2,4-D (3.8 lb/gal formulation) or 1 pt/A of Banvel or Clarity to the rosette stage in fall or early spring before the plants send up the flower stalk is recommended. For Canada thistle, the ideal timing would be during periods of active growth after weeds have reached the bud stage in mid- to late summer, but before killing frost. At this time, the plants have maximum leaf area to absorb herbicides and begin moving carbohydrates into the rootstocks. These stored carbohydrates allow the plants to survive winter and emerge again in the spring. Herbicide movement into these rootstocks is facilitated by this process. Application of 4 pt/A of 2,4-D (3.8 lb/gal formulation) or of 2 pt/A of Banvel or Clarity are recommended for Canada thistle suppression in grass pastures. Repeated applications of these herbicides would likely be needed to bring this tough weed under control.
With both 2,4-D and Banvel/Clarity, label instructions specify grazing and harvesting restrictions for pasture situations. Lactating dairy animals should not graze 2,4-D treated areas for 7 days following application and meat animals must be removed from 2,4-D treated areas for 3 days before slaughter if less than 14 days have elapsed since treatment. Lactating dairy animals should not graze for 7 days after treatment with up to 1 pt/A, and for 21 days after 2 pt/A of Banvel or Clarity. Meat animals should be removed from areas treated with Banvel or Clarity 30 days before slaughter. Applications made at the end of the grazing season in late summer or early fall can minimize concerns about these grazing restrictions.